Referring to vinyl polymers produced by radical polymerization, in contrast to those polymers which are produced by ionic polymerization or polycondensation, few polymers having functional groups, particularly vinyl polymers at molecular chain terminus such functional groups, are available as of today. Among such vinyl polymers, (meth)acrylic polymers have certain characteristics not shared by polyether polymers, hydrocarbon polymers or polyester polymers, such as high weathering resistance and transparency, and said (meth)acrylic polymers having an alkenyl or crosslinking silyl group in the side chain have been utilized in weather-resistant coatings, among other applications. Meanwhile, the control of polymerization reaction of acrylic polymers is handicapped by side reactions and the introduction of a functional group into the molecular chain terminus, for instance, is extremely difficult.
Should it be possible to produce an alkenyl group-terminated vinyl polymer by an expedient method, cured products having superior physical properties as compared with those obtainable from ones having crosslinking side-chain groups could be obtained. From this point of view, attempts to establish such a technology have been made by many, workers to this day but actually it has proved difficult to produce such polymers on a commercial scale. By way of illustration, Japanese Kokai Publication Hei-1-247403 and Japanese Kokai Publication Hei-5-255415 disclose processes for synthesizing alkenyl-terminated (meth)acrylic polymers using an alkenyl group-containing disulfide as the chain transfer agent.
Japanese Kokai Publication Hei-5-262808 discloses a process for producing an alkenyl-terminated (meth)acrylic polymer which comprises synthesizing a vinyl polymer having a hydroxyl group at both termini by using a hydroxyl-containing disulfide and, then, converting the terminal hydroxyl group to an alkenyl group by utilizing the reactivity of the hydroxyl functional group.
Japanese Kokai Publication Hei-5-211922 discloses a process for producing a silyl-terminated (meth)acrylic polymer which comprises synthesizing a vinyl polymer having a hydroxyl group at both termini by using a hydroxyl-containing polysulfide and converting the terminal hydroxyl groups to silyl groups by utilizing the reactivity of the hydroxyl functional group.
By these processes, however, it is difficult to certainly introduce functional groups into both termini of the molecular chain, hence to give cured products having satisfactory characteristics. In order that a functional group may be introduced into both termini with certainty, the chain transfer agent must be used in a large amount, which is disadvantageous process-wise. Furthermore, since the reaction involves a standard radical polymerization reaction in these processes, the molecular weight and molecular weight distribution (the ratio of weight average molecular weight to number average molecular weight) of the product polymer cannot be easily controlled.
In view of the above conventional technology, the inventors already did several inventions relating to vinyl polymers having various crosslinking silyl groups at its terminus, processes for producing the polymers, curable compositions and uses [e.g. Japanese Kokai Publication Hei-11-080249, Japanese Kokai Publication Hei-11-080250, Japanese Kokai Publication Hei-11-005815, Japanese Kokai Publication Hei-11-116617, Japanese Kokai Publication Hei-11-116606, Japanese Kokai Publication Hei-11-080571, Japanese Kokai Publication Hei-11-080570, Japanese Kokai Publication Hei-11-130931, Japanese Kokai Publication Hei-11-100433, Japanese Kokai Publication Hei-11-116763, Japanese Kokai Publication Hei-9-272714, Japanese Kokai Publication Hei-9-272715, etc.]
For example, a vinyl polymer having a crosslinking silicon-containing group (hereinafter referred to sometimes as “crosslinking silyl group”) comprising a hydroxyl or hydrolyzable group bound to a silicon atom and capable of siloxane bonding or a cured product obtainable from this has excellent heat resistance and weather resistance, therefore these can be used in various fields such as architectural elastic sealants and composite-glass sealants, coatings, sealing materials or members and so on.
However, cured products available from such a vinyl polymer having a crosslinking silyl group tend to have the drawback of a conflict between hardness and surface tackiness (also referred as to tacky or residual tack); that is to say, products which is required to be low hardness, i.e. elastic, express a greater residual tack on its surface and tend to be easily soiled. For example, in use as an architectural sealant, the residual tack attracts soil and dust to the surface to adversely affect the appearance of buildings. The first aspect of the present invention, therefore, has for its object to reduce the surface tackiness (residual tack) of cured products obtainable from vinyl polymer having a crosslinking silyl group.
Meanwhile, cured products obtainable by using a vinyl polymer having a crosslinking functional group such as a crosslinking silyl group as the curable component have satisfactory heat resistance and weather resistance and exhibit good coatability when a coating is applied thereon. However, when a well-known plasticizer of comparatively low molecular weight, such as a phthalic acid ester, is used for the purpose of lowering the viscosity of the formulation, the gradual elution of the plasticizer by heat or rain water from the cured product makes it difficult to maintain the initial physical properties of the product for a long time. The additional disadvantage is that when a coating known as “alkyd coating” is applied, the coating is hard to be dried and cure easily.
Therefore, the second aspect of the present invention has for its object to reduce the surface tackiness (residual tack) of the cured product obtainable by using a vinyl polymer having a crosslinking silyl group as the curable component to thereby minimize the settlement of dust thereon while upholding the satisfactory mechanical properties of the cured product and, at the same time, improve the coatability of the product, to an alkyd coating. The third aspect of the present invention has for its object to maintain the heat resistance and weather resistance of the cured product available from the crosslinking functional group-containing vinyl polymer over a protracted time period and, at the same time, improve the coatablilty of the cured product to an alkyd coating.
Meanwhile, in order to impart flexibility to such cured products through reduction in the modulus thereof, it is generally necessary to increase the molecular weight of the polymer but this entails an increased viscosity of the polymer, thus detracting from workability. An alternative approach comprises lowering the rate of introduction of a crosslinking silyl group instead of increasing the molecular mass of the polymer but, in this case, the uncrosslinked fraction is increased to cause a reduced cure speed and a reduced gel fraction of the cured product, thus exerting adverse effects on physical properties other than flexibility. Therefore, in order to reduce viscosity while sustaining flexibility, it is common practice to add one of various plasticizers.
As such plasticizers, there can be mentioned aromatic carboxylic acid esters, aliphatic carboxylic acid esters, phosphoric acid esters, glycols, epoxy plasticizers and chlorinated paraffin, among others. However, these plasticizers have migrating properties so that when used for sealants or adhesives, they tend to cause such troubles as fouling at and around sealed joints, adverse influences on adhesion, and a decrease in flexibility due to extraction of the plasticizer on prolonged curing. The fourth aspect of the present invention, therefore, has for its object to improve workability in a compounding stage or a curable composition application stage, to impart flexibility to cured products, and inhibit adverse influences of plasticizer migration.
As means for reducing the crosslinking silyl group content of a vinyl polymer without reducing the amount of introduction of the crosslinking silyl group to thereby impart flexibility to cured products through a reduction in modules, Japanese Kokai Publication Sho-61-34067 and Japanese Kokai Publication Sho-64-9268, among others, disclose the technology involving addition of a compound having one silanol group per molecule and/or a compound capable of reacting with moisture to give a compound containing one silanol group per molecule (hereinafter sometimes these are collectively referred to as “silanol-containing compound”).
However, the organic vinyl polymer containing at least one reactive silicon functional group per molecule as described in Japanese Kokai Publication Sho-61-34067 is produced by the standard free radical polymerization reaction using a chain transfer agent and, therefore, has a high viscosity as well as the disadvantage that in order to attain flexibility while retaining a high gel fraction, it is necessary to use an unsaturated organosilicon monomer in a large amount and a silanol-containing compound also in an increased amount. The fifth aspect of the present invention, therefore, has for its object to provide a curable composition which, despite its low viscosity, gives a cured product with a high gel fraction, low residual tack, low modulus, high elongation, and good flexibility.